盐湖相有机质富集主控因素与模式——以柴西盆地新近系上新统下油砂山组灰色泥岩为例

Main controlling factors and models of organic matter enrichment in salt lake facies:A case study of gray mudstone of Lower Youshashan Formation in western Qaidam Basin

盐湖相页岩油是非常规油气勘探的潜在接替目标,然而盐湖相有机质富集主控因素与模式仍未探究清楚。研究基于柴达木盆地南翼山地区新近系上新统下油砂山组灰色泥岩样品,通过总有机碳含量、主量、微量及稀土元素测试,重建下油砂山组灰色泥岩沉积期古环境,探索盐湖相有机质富集机制。研究结果表明:下油砂山组灰色泥岩沉积环境由氧化转至贫氧;气候条件较为干旱,且干旱程度逐渐加剧;沉积水体呈咸水状态偶见半咸水,下油砂山组下段盐度较为稳定,古生产力较低,下油砂山组上段盐度波动较大,生产力升高至中等条件;陆源输入整体偏低且下段高于上段,沉积速率较慢,处于低能—停滞水体环境,上段更差的水动力不利于有机质的迁移和保存,下段相较而言适宜的水动力可以促成有机质良好的富集。整体而言,下段有机质富集主控因素为保存条件,上段主控因素为古生产力,且整个沉积时期经历了5次有机质富集的峰值,盐湖相有机质富集模式动态变化的驱动因素可能为频繁波动的古气候。

Shale oil in saline lacustrine basins is a potential replacement target for unconventional oil and gas exploration,but the main controlling factors and models for the organic matter enrichment(OME)remain unclear.In order to reconstruct the palaeoenvironment during the organic-rich mudstone deposition and explore the OME mechanism in saline lacustrine basins,samples from the Lower Youshashan Formation in the Nanyishan area of Qaidam Basin were used to carry out experiments,such as TOC,major,trace,and rare earth elements.The results show that:The main controlling factor of the OME in the lower and upper parts of the Lower Youshashan Formation is preservation condition,and paleoproductivity,respectively.There are five OME peaks:Peak 1 is“mixed model”,which is controlled by preservation condition and paleoproductivity,peak 2 is“preservation model”,which is controlled by preservation condition,peaks 3 and 4 are“productivity model”which are controlled by paleoproductivity,and peak 5 is“mixed model”,which is controlled by multiple factors.Furthermore,dynamic changes of the driving factors for the OME may be the frequent fluctuations of paleoclimate.